As is known in today's electronics industry, there is an increasingly marked trend towards integrating a number of devices in a single multifunctional system. For example, the field of mobile telephones is undergoing an extremely rapid evolution, and one of the most important lines of development envisages the integration of a miniaturized videocamera in a cellular phone.
For this purpose, it is necessary to design optical sensors which, on the one hand, present overall dimensions that are as small as possible and are simple and inexpensive to manufacture, and which, on the other hand, will have optical characteristics that will not degrade the quality of the images that are detected. For this reason, the optical sensors must also be protected against contamination by external agents, such as dust and humidity, which could lead to irreparable damage. In particular, optical sensors are normally sealed inside hollow encapsulation structures. For greater clarity of exposition, we shall refer to FIG. 1, where a sensor 1 is illustrated, for example, an array CMOS sensor, formed in a die of semiconductor material obtained by cutting a wafer (not shown herein). The sensor 1 is encapsulated in a protective structure 2, which comprises a base 3, for example, formed by a lamina of pre-set thickness, and a supporting frame 4 of ceramic or plastic material, formed on the base 3 and having a depth greater than that of the sensor 1. In greater detail, the supporting frame 4 and the base 3 define a cavity 5, in which the sensor 1 is housed. In addition, the cavity 5 is sealed by means of a plate 6 of transparent material, preferably glass, which is bonded to the supporting frame 4, at a distance from the sensor 1. The plate 6 protects the sensor 1 from contaminating agents, without altering the optical properties of the incident light beams. Connection lines 7 enable the contacts of the sensor 1 to be brought outside the protective structure 2, through the base 3, said contacts being formed on the face of the sensor 1 facing the plate 6.
Known processes for encapsulating optical sensors present, however, various drawbacks.
In the first place, the overall dimensions of the hollow protective structures are considerable as compared to the size of the optical sensor and cannot be reduced beyond a certain limit. In fact, the supporting frame 4 must be quite thick in order to enable bonding of the plate 6. Furthermore, between the supporting frame 4 and the sensor 1, it is necessary to provide a region of free-space, in which the connection lines 7 are formed. In practice, the protective structure 2 has a width and a length that may be even twice those of the sensor 1. It is, therefore, evident that the degree of miniaturization of devices incorporating sensors encapsulated in hollow protective structures is accordingly limited.
In the second place, it is very likely for impurities to penetrate inside the cavity 5 when the sensor 1 is being manufactured and mounted in the protective structure 2. In fact, the sensor 1 can be encapsulated only after the initial wafer has been divided into individual dies. On the other hand, it is known that, during the steps of cutting of the wafers, a considerable amount of dust is created. Furthermore, even in the most well-controlled production areas, there is inevitably present some dust which, during the steps of conveying, handling, and assembly, may deposit on top of the sensor 1. Given that, in this way, the quality of the images that can be detected may easily be impaired, many sensors have to be eliminated and, in practice, the overall output of the manufacturing process is other than optimal.
A further drawback is due to the wide tolerances that are to be envisaged in the steps of manufacturing the protective structure 2 and encapsulating the sensor 1. Since these tolerances render the process far from easily repeatable, successive processing steps must be optimized for each individual piece. In particular, in the manufacture of miniaturized video cameras, an optical assembly is coupled to the encapsulated sensor 1. More specifically, the optical assembly is bonded to the plate 6. On account of the intrinsic imprecision of the protective structures 2, it is not sufficient to use exclusively mechanical centering references prearranged during the manufacture of the sensor 1 and of the protective structure 2. Instead, it is necessary to carry out a laborious process for aligning the optical assembly with respect to the surface of the sensor 1, so that the optical axis will be orthogonal to said surface, and, subsequently, for focusing the optical assembly. As mentioned above, this process must be carried out for each individual piece manufactured and has a marked incidence on the overall cost of manufacture.